The more something is like ourselves, the harder it is to attack it with chemicals. Our cells are very similar in nature to a lot of protozoan parasites, so the drugs that may treat them are pretty toxic to ourselves. Bacteria and fungi are much less like our cells, and they have a lot of unique features such as peptidoglycan or ergosterol in the cell wall, therefore they can be attacked with chemicals that are less toxic. Animals, such as insects and nematodes, that have diverged significantly away from our common origin, have evolved new features that we do not share, and we can attack those. Some organisms of interest to (veterinary) medicine, such as Leishmania and Pythium, are very difficult to treat.